1. Field of the Invention
This invention relates to improved systems and methods for controlling emissions from internal combustion engines. In one specific aspect, the present invention relates to improving catalyst converter light-off performance following engine cold start. In another specific aspect, the present invention relates to efficient and effective control of emissions at highway speeds.
2. Brief Description of the Prior Art
Automotive emissions are still a major environmental problem despite major advances in emissions control resulting from the use of catalytic converters. First and most important, pollution is not controlled during the initial sixty to ninety or more seconds of operation after cold starting (weather dependent) required to heat the converter catalyst to an effective operating temperature. In addition, emissions are not controlled at typical high load highway speeds (above the range covered by the Federal Test Procedure (FTP)) at which engines must also run sufficiently fuel rich that effective and efficient operation of the catalytic converter is impaired by the lack of sufficient oxygen. As is well established in the art, an engine must operate within about one percent of the stoichiometric fuel/air ratio if all three exhaust pollutants, hydrocarbons, carbon monoxide, and nitrogen oxide are to be efficiently reacted by the catalytic converter. This typically requires closed loop control using an exhaust gas lambda sensor. Even with as little as one to three percent excess fuel, hydrocarbon emissions can be excessive. No.sub.X control is impaired with less than one percent excess air.
To meet the emission levels required by new regulations such as the California ULEV standards, it is critical that effective and efficient operation of the catalyst be achieved in less than about twenty or thirty seconds after engine starting, especially with larger engines. However, in present converters heating of the catalyst to an effective temperature by hot engine exhaust gases typically takes more than sixty seconds, even with close coupling of the catalyst to the engine. Provided sufficient oxygen is present in the exhaust, the required fast light-off can be achieved by using electrically preheated catalysts, close coupled catalysts with much higher precious metal loadings to reduce light-off temperature or most advantageously the fast light-off catalysts of my U.S. Pat. No. 5,417,933 which have demonstrated fast light-off even without close coupling to the engine. Typically however, during the first thirty or so seconds of operation, the engine operates rich (ie with insufficient air to consume all the fuel, typically with lambda values much lower than 0.95 or even 0.85) resulting in exhaust gas with insufficient oxygen for adequate catalytic conversion of hydrocarbons and carbon monoxide. To achieve efficient conversion of all three pollutants, stoichiometry must be very close to a lambda value of one, with a lambda even as high as about 1.01 (1% excess air) NO.sub.X conversion is minimal and with a lambda of 0.98 (excess fuel) hydrocarbon and carbon monoxide conversion are seriously impaired. Thus, even with the various means which have been developed for rapid catalyst light-off, only partial conversion of hydrocarbons is possible without the presence of sufficient oxygen in the exhaust gases, ie. exhaust gas lambda must be higher than about 0.98.
Efficient means to supply "sufficient" oxygen (hereinafter air) in the exhaust stream has not been developed. For any catalyst to operate at its required conversion level sufficient air must be available. "Sufficient air" is air quantities sufficient to allow achievement of required conversion levels. "Suitable air" is sufficient air that limits cooling of the exhaust gases. Current technology typically provides sufficient air by means of an air pump to add the air needed for conversion to the exhaust gas. Addition of air to the exhaust is difficult to regulate and requires additional engine hardware. Moreover, the air is at ambient temperature and thus tends to cool the exhaust stream to the catalyst, in addition there is often either too little air added to support adequate conversion or too much such that NO.sub.X conversion is impaired.
To avoid the need for air addition, there have been efforts to limit the initial period of low lambda engine operation to less than thirty seconds. Although there has been some success in reducing the duration of low lambda operation for mild ambient air temperatures, engines must start even at winter temperatures and still achieve low emissions. In addition, emissions must also be controlled at highway speeds where very low lambda operation is often needed for engine cooling.
The present invention meets the need to control internal combustion engine emissions during periods of rich operation by providing a method of engine operation which utilizes the engine itself to provide oxygen for catalyst operation during such rich burn operation by balancing rich burn cylinder firings with compensating lean burn firings. The invention offers the advantage over the prior art of allowing for addition of more suitable air to the exhaust, air at a temperature greater than ambient and with greater regulation. Thus, the present invention allows much more rapid light-off of fast light-off or close coupled converter catalysts after engine starting, and also allows for emissions control at high speed/high power conditions for which low lambda operation provides required engine cooling. The latter aspect of this invention is especially important in view of the new USO6 standards which require emissions control at speeds above those for the present FTP test. In the method of the present invention as applied to engine cooling, not only is an engine exhaust composition provided which allows suitable emissions control but all cylinder firings impose less thermal loading than if operated at stoichiometric ratios.